1. Field
The present disclosure relates generally to optical communications, and more particularly to using the carrier phase angles recovered from the coarse phase recovery stage to estimate and remove carrier frequency offset for a carrier system.
2. Brief Description of the Related Art
In order to meet growing capacity demands in core optical networks, spectrally efficient techniques, such as digital coherent detection, have attracted recent attention. These techniques allow the use of advanced modulation formats; especially M-ary quadrature amplitude modulation (QAM) modulated systems. However, one major challenge in implementing high-performance coherent detection is in accurate phase and frequency offset recovery, which is caused by intrinsic laser phase noise and signal-local oscillator frequency offset. As a result, for high-order M-QAM modulation formats (where M>4), tolerance to laser phase noise decreases as the modulation level increases, because the Euclidean distance decreases (Yu, X. Zhou and J., “Multi-level, Multi-dimensional Coding for High-Speed and High Spectral-Efficiency Optical Transmission.” to be published in the August issue of J. Light wave Technology, 2009). In particular, while frequency offset is relatively slow-changing, phase drift caused by laser phase noise (characterized by laser linewidth) is fast-changing. Given the small tolerance of high-order M-QAM systems to phase and frequency noise, the quality of phase tracking significantly influences performance of the communication system.
Presently, there are three published carrier phase recovery schemes. The first is a decision-directed digital feedback loop (Irshaad Fatadin, David Ives, Seb J. Savory., “Compensation of Frequency Offset for Differentially encoded 16- and 64-QAM in the presence of laser phase noise.” IEEE Photonics Technology Letters. Feb. 1, 2010, p. 2010; H. Louchet, K. Kuzmin, and A. Richter., Improved DSP algorithms for coherent 16-QAM transmission, Brussels, Belgium: Tu.1.E.6, 2008. Proc. ECOC'08. pp. Sep. 21-25, 2008; A. Tarighat, R. Hsu, A. Sayed, and B. Jalali., Digital adaptive phase noise reduction in coherent optical links, J. Lightw. Technol., vol. 24, no. 3, March 2006, pp. 1269-1276). Since this method relies on negative feedback, its performance depends heavily on the ability of previous samples to be relatively current, which places demands on the sampling frequency. This is especially a problem in parallel and pipeline architectures, in which sampling is both sparse and delayed.
The second method uses a classic feed-forward phase correction technique based on an Mth-power Viterbi-Viterbi algorithm, in which the phase quadrant information is deliberately removed to calculate phase error (Seimetz, M., “Laser linewidth limitations for optical systems with high-order modulation employing feed forward digital carrier phase estimation.” San Diego, Calif.: OTuM2, Feb. 24-28, 2008. Proc. OFC/NFOEC). However, this method can only be applied to certain constellation points having equal phase spacing, and therefore only a small subset of incoming signals can be used—this again reduces the linewidth tolerance of the system.
A third method proposes using a blind exhaustive phase search to find phase error based on the phase distance to the nearest constellation point, for a collection of points (T. Pfau, S. Hoffmann and R. Noé., Hardware-Efficient Coherent Digital Receiver Concept With Feed-forward Carrier Recovery for M-QAM Constellations, Journal of Lightwave Technology, Vol. 27, No. 8, Apr. 15, 2009). While this method is both feed-forward and high-performing, it requires high complexity to process a large collection of points simultaneously. In addition, because of the need to process in parallel, each group of computations required to process the collection of points must be repeated for each parallel branch. Therefore, though this method is high-performing, it is not feasible to implement.